The stroke volume of the heart is defined as the volume of blood expelled by a ventricle in a single beat. It is equal to the difference between end diastolic volume and end systolic volume. The importance of stroke volume in determining the state of the heart is evident in that the cardiac output, i.e., the total volume of blood pumped by a ventricle during a period of time, is equal to the product of the heart rate and the stroke volume. The stroke volume may be increased either by decreasing the end systolic volume, implying increased myocardial shortening. or by increasing end diastolic volume. Increases in ventricular distention result in greatly increased wall tension to develope the same intraventricular pressure during ejection.
In normal human subjects with healthy hearts, the stroke volume of the heart remains relatively constant over a wide range of exertion, from minimal activity to high physical exertion. The increases in cardiac output under stress are due primarily to increased heart rate, at least up to a point. Also, in normal human subjects, increases in stroke volume are reported only under maximal exercise conditions as the heart rate tends to level off. In contrast, with patients suffering from third degree heart block where a fixed rate cardiac pacer is determining the heart rate, increased cardiac output during exertion is due principally to increased stroke volume. However, stroke volume cannot increase by more than a factor of 2 to 21/2 which limits the exercise capabilities of these patients.
Because stroke volume, in individuals with a fixed heart rate due to heart block, is a useful indicator of cardiovascular load, and since studies indicate a constancy of stroke volume under sub-maximal exercise, it would be advantageous to have a cardiac pacer having a variable rate and a means for varying that rate as a function of stroke volume. In such an arrangement, during exercise, the pulse generator would be designed to sense the increase in stroke volume due to changes in physiologic demand and would function to increase the pulse rate so as to bring the stroke volume parameter back to its original value.
In a paper entitled "Continuous Measurement of Ventricular Stroke Volume by Electrical Impedance" published in the Cardiovascular Research Center Bulletin, Volume 4, No. 4, April-June 1966, Pages 118 through 131, L. A. Geddes and his associates at the Baylor University College of Medicine in Houston, Texas reported on an approach for measuring stroke volume by sensing changes in impedance between two spaced electrodes disposed within the ventricular cavity. It was theorized that the blood in the ventricle constitutes an electrical conductor of irregular and changing shape such that spaced electrodes placed in the ventricle could be used to sense instantaneous impedance variations observed between the electrodes as the blood fills and leaves the ventricle.
In the Knudson et al application Ser. No. 170,947, filed July 21, 1980, now U.S. Pat. No. 4,313,442 there is described a cardiac pacer whose rate of generation of pacer pulses is controllable as a function of changes in the detected P-wave rate. In that arrangement, a lead having a stimulating electrode at its distal tip is positioned so that the electrode abuts the apex of the ventricle. That lead has further sensing electrodes disposed a predetermined distance proximally of the tip of the stimulating electrode so as to be located near the upper right wall of the atrium. The atrial (P-wave) activity picked up by the sensing electrodes is processed in suitable circuitry and used to vary the interpulse interval of a demand-type cardiac pacer. In that the P-wave activity is indicative of physiologic demand, the variable rate pacer of the aforesaid Knudson et al patent permitted the stimulating rate to be altered as a function of body needs.
When the present invention is used in conjunction with a cardiac pacer having a variable rate capability, P-wave activity is no longer an indicator of physiologic demand, but instead, changes in stroke volume are detected and a signal is developed which is proportional to those changes. By applying that signal as a control signal to the timing circuit of a demand-type cardiac pacer, the pacer pulse generator will output stimulating pulses in accordance with the physiologic demand indicated by stroke volume changes. The method and apparatus of the present invention as applied to a cardiac pacer involves varying the ventricular pacing rate in such a fashion as to minimize changes in stroke volume. Thus, during exercise, the sensing circuitry to be described senses changes in ventricular volume or stroke volume and then alters the operating parameters of the utilization device. For example, in a cardiac pacer application, increases in stroke volume may be sensed and the resulting control signal may be used to increase the pulse rate so as to bring the stroke volume back to its original value. This may be done either with respect to an absolute reference stroke volume or, as in the system of the aforereferenced Knudson patent, by sensing only changes in stroke volume and altering the rate to minimize the changes.